Gathering for sickly Oscar's leaving (5)
I believe the answer is:
frill
'gathering' is the definition.
(I've seen this before)
'for sickly oscar's leaving' is the wordplay.
'sickly' becomes 'ill' (similar in meaning).
'oscar' becomes 'o' (phonetic alphabet: alpha, bravo, charlie etc.).
'leaving' indicates named letters should be taken away.
'for'+'ill'='forill'
'forill' with 'o' removed is 'FRILL'.
(Other definitions for frill that I've seen before include "Gathered strip as a trimming" , "Ruche; ruff" , "A decorative gathering of fabric" , "Shared feature of Mr Punch, Richard III" , "Tuck" .)